Revelation of bimolecular tautomerization induced by the concerted and radical interactions in lignin pyrolysis

Abstract

Bimolecular interactions play crucial roles in lignin pyrolysis. The tautomerization of key intermediates has a significant impact on the formation of stable products, whereas bimolecular tautomerization has been rarely clarified. In the present work, the bimolecular tautomerization mechanism induced by both concerted and radical interactions was proposed and carefully confirmed. A characteristic β-O-4 lignin dimer, 2-phenoxy-1-phenylethanol (α-OH-PPE), was used as the model compound to reveal two representative keto-phenol and enol-keto tautomerism mechanisms, based on theoretical calculations combined with pyrolysis experiments. The results indicate that the unimolecular tautomerism as the rate-determining step limits product generation, due to fairly high energy barriers. While the free hydroxy compounds and radicals derived from initial pyrolysis can further initiate bimolecular tautomerism reactions through the one-step concerted hydroxyl-assisted hydrogen transfer (hydroxyl-AHT) and two-step radical hydrogen abstraction interactions, respectively. By alleviating and even avoiding the large ring tension of tautomerism, the unstable tautomers (2,4-cyclohexadienone and 1-hydroxystyrene) can be rapidly tautomerized into stable phenol and acetophenone with the help of intermolecular interaction. Benefitting from the significant advantage of retro-ene fragmentation in breaking the β-O-4 bond to form tautomers, a large amount of stable phenolic and ketone products can be generated following bimolecular tautomerization in the pyrolysis of β-O-4 linked lignin.